Add default implementation for IDocument::fallbackSaveAs(Path|FileName)

The methods are only relevant for documents without a filePath, and
there was a mix of different irrelevant implementations present in
subclasses.

Change-Id: I4f57d306e5ddd913974cfe6ed0b4db062eb907a1
Reviewed-by: David Schulz <david.schulz@theqtcompany.com>

src/plugins/qmakeprojectmanager/qmakeproject.cpp

@@ -104,9 +104,6 @@ public:
 
     bool save(QString *errorString, const QString &fileName, bool autoSave) override;
 
-    QString fallbackSaveAsPath() const override;
-    QString fallbackSaveAsFileName() const override;
-
     bool isModified() const override;
     bool isSaveAsAllowed() const override;
 
@@ -266,16 +263,6 @@ bool QmakeProjectFile::save(QString *, const QString &, bool)
     return false;
 }
 
-QString QmakeProjectFile::fallbackSaveAsPath() const
-{
-    return QString();
-}
-
-QString QmakeProjectFile::fallbackSaveAsFileName() const
-{
-    return QString();
-}
-
 bool QmakeProjectFile::isModified() const
 {
     return false; // we save after changing anyway